Synapse previously did not correctly cap the max depth of an event to
the max canonical json int. This can cause ordering issues for any
events that were sent locally at the time.
This background update goes and correctly caps the topological ordering
to the new `MAX_DEPTH`.
c.f. GHSA-v56r-hwv5-mxg6
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Co-authored-by: Andrew Morgan <1342360+anoadragon453@users.noreply.github.com>